1. Field of the Invention
The present invention relates to microspherical sintered bodies of hydroxyapatite and a new process for preparing same as well as a chromatographic packing material comprising the microspherical sintered bodies of hydroxyapatite. More particularly, the present invention relates to microspherical sintered bodies of hydroxyapatite which are excellent in mechanical strength, durability and reproducibility and suitable as a packing material (sorption agent) for liquid chromatography and a process for preparing same according to spray-firing as a new technical means as well as a chromatographic packing material comprising such microspherical sintered bodies.
2. Description of the Prior Art
From the past, hydroxyapatite possessing a unique performance as an adsorbent for bio-macromolecules, fluorine ion, heavy metal ions, etc. has been watched as a packing material (sorption agent) for liquid chromatography for the separation and purification of proteins, enzymes, nucleic acids and the like biological substances. In recent years, hydroxyapatite possessing such biocompatibility is utilized also as bio-ceramic materials such as artificial bones and teeth and has a bright prospect in this art.
The fact that proteins can be adsorbed to calcium phosphate gel is known from the past. In chromatographic use, however, such gel gives too compact columns so that the gel fails to give a test solution a flow speed practical for column chromatography, thus making it difficult to use such gel for a chromatographic purpose. In case of using such calcium phosphate gel, therefore, it had to be mixed with a small amount of an auxiliary substance capable of facilitating passage of the solution through the column, such as Super-Gel before packing. However, such auxiliary substance may adsorb biomacromolecules, thus making the result complicated. For the above reasons, the calcium phosphate gel is quite undesirable as a packing material for column chromatographic analysis of bio-macromolecules.
Since the technique for the preparation of hydroxyapatite utilizable for column chromatography was developed by Tiselius et al. [Arch. Biochem. Biophys., 65, 132-155 (1956)], hydroxyapatite for column chromatographic use has been prepared for a long time up to date according to their processs or various processes wherein their process is modified. According to these processes, however, hydroxyapatite is obtained in the form of plate-like crystals or agglomerates of microcrystals and so has such a shortcoming that it is inferior in mechanical strength and tends to be destroyed during the packing operation and measurement. Thus, chromatographic characteristics of hydroxyapatite are changed according to the packing method used or in the course of the measurement so that problems arise in durability of the packing material and in trustfulness of the measurement.
In recent years, a process for producing microspherical hydroxyapatite was proposed to overcome the above mentioned shortcoming, utilizing the so-called spray-drying method which is widely used for manufacturing granules of a powdery substance (Japanese Laid-open Patent Appln. Nos. Sho. 62-206445 and 62-230607). According to the process disclosed in Japanese Laid-open Patent Appln. No. 62-206445, microcrystals of hydroxyapatite having a diameter of less than 1 .mu.m as primary particles are physically coagulated by spray drying to form substantially spherical particles of 1-10 .mu.m in diameter as second particles. Japanese Laid-open Patent Appln. No. Sho. 62-230607 discloses a process for preparing spherical agglomerates of apatite wherein a gelled hydroxyapatite slurry is sprayed into an atmosphere kept at 100.degree.-200.degree. C. to form spherical agglomerates of hydroxyapatite having a diameter of 1-10 .mu.m. In case the spherical hydroxyapatite particles obtained according to such prior art processes are subjected to classification by screening to collect particles of a definite particle size as a packing material for liquid chromatography, the spherical particles tend to be destroyed because of their poor mechanical strength and will be broken to pieces when packed densely in a column under high pressure. Consequently, the spherical hydroxyapatite particles formed by spray drying have to be brought to a heat treatment carried out at a high temperature for a long period of time in order to impart to them mechanical strength sufficient enough to withstand high pressure on packing. Under such severe heat treatment, however, there arises a problem that the spherical particles tend to be bonded to one another in a mutually fused state to form partially solid state granules. Thus, the prior art processes involve a number of problems not only in the preparation of spherical hydroxyapatite particles but also in the use of the particles as a packing material for chromatographic purposes. Thus, there was a great demand for developing spherical sintered bodies of hydroxyapatite particles which are excellent in mechanical strength and chromatographic characteristics and easy in handling as well as a process for preparing same.
Under the above mentioned circumstances, one of the present inventors previously developed a process for preparing microspherical hydroxyapatite particles according to a specific spray-pyrolysis technique (Japanese Laid-open Patent Appln. Nos. Sho. 61-146704 and 61-201612) as well as a packing material for chromatographic use comprised of microspherical hydroxyapatite particles possessing high mechanical strength and a process for preparing same (Japanese Laid-open Patent Appln. No. Sho. 62-67451). In these processes, a calcium compound and a phosphorus compound have to be dissolved, without permitting the formation of any precipitate, in a solution to be sprayed. In practice of these processes, therefore, some limitations were necessary in selection and combination of the starting materials.
The processes described just above are generally regarded desirable for the preparation of microspherical hydroxyapatite particle for chromatographic use. However, there is still room for improving the processes to obtain microspherical hydroxyapatite particles with desirable properties in a simpler operation.